1. Field of The Invention
The invention relates to the field of power supply back-up systems, and in particular, to the testing of a power supply back-up system.
2. Background Information
In critical applications requiring electrical power, such as computer systems, it is desirable to ensure against an unexpected power outage which could have an adverse effect on the electrical equipment being powered or the task the equipment is performing. In order to provide uninterrupted operation of the electrical equipment when there is a failure of a main power source, it is known to provide a back-up power system, such as an uninterruptible power supply (UPS) (see e.g. Stich et al. U.S. Pat. No. 5,182,518).
There are known a number of different types of back-up power systems suitable for use in the home or office, all of which generally rely on at least one electrical energy storage battery as a source of back-up power. Besides the traditional storage battery based back-up power system, other types of back-up power systems, using electrical generators alone or in combination with storage batteries, for example, are also known. These are generally more suitable to heavy industrial uses with large power requirements, although small portable generators which operate on ordinary gasoline are now available for home or small office use.
Of course, larger generator based systems use turbine generators which require fossil fuels to produce the steam that powers them, or consume some other source of bulk stored energy, such as the kinetic energy of water stored in a hydroelectric dam to power the generator, and are therefore not generally found in the home or office in a typical back-up power system. Solar powered steam turbine electrical generators are also known in which solar energy is focused on a water filled pipe or vessel to produce steam which is used to turn a generator.
Solar cells, also called photovoltaic cells, which rely on solar energy input to produce a DC (direct current) electrical output, are also known as a primary or back-up source of electrical energy. Solar cells, alone or in combination with storage batteries, have particular usefulness in remote areas where traditional power sources, such as electrical utility lines or electrical generators, are not suitable. Also known are wind powered and tide powered electrical generators.
Fuel cells are also known as a primary or back-up source of electrical energy. For a number of reasons not relevant to this disclosure, fuel cells are particularly suitable for use in spacecraft and the like. Another primary or back-up source of energy can be found in nuclear materials which have been used in power supplies in submarines and spacecraft, for example, in the past.
Suffice it to say that, depending on the requirements of the task to which the power system is being applied, any of the above-mentioned energy sources, and other similar sources not mentioned, may be used as a primary and/or back-up power supply.
One type of back-up power system which uses one or more electrical storage batteries is referred to herein as a battery back-up unit (BBU). In a BBU, a direct current (DC) voltage and current required by the electrical equipment being backed-up, also referred to as the load, may be provided directly by the one or more batteries upon failure of the main power source in a power supply. Upon failure of the main power source in the power supply, the one or more batteries are automatically connected to the output to power the load. The one or more batteries are connected in an electrical circuit referred to herein as the battery path. For an example of a known battery back-up power system, see Chen U.S. Pat. No. 5,528,149 or Chen et al. U.S. Pat. No. 5,726,573.
Known back-up power systems may use one or more batteries in combination with an inverter to provide an alternating current (AC) output from the DC provided by the batteries (see e.g. Stich et al. U.S. Pat. No. 5,182,518). The AC back-up output is supplied directly to the equipment being backed-up if it operates on an AC input, or after the inversion, rectified into a DC voltage through additional circuitry (see e.g. Collins et al. U.S. Pat. No. 4,916,438).
Thus, back-up power circuitry having a battery (or other DC source) can be connected to provide an AC source of back-up power through the use of an inverter, or to provide a DC source of back-up power, depending on the requirements of the electrical equipment being backed-up. Some computer systems may be provided with an optional battery back-up unit (BBU) inside the computer cabinet which is connected to provide a DC source should the DC output of the main power supply fail, either because of a main power supply component failure, or because of a loss of power input to the main power supply (e.g., an electrical utility outage).
To ensure that a back-up power system, such as a battery back-up power system (BBU), is in good operating condition such that it will function properly when needed, it is prudent to monitor/test the back-up power system from time to time. In a BBU, for example, a battery path test should be done to determine that all the components in the path from the battery to the circuits being backed-up, i.e., the load, work properly.
A path test is typically done at the beginning of a battery capacity test or when a system is first powered on. There are a number of different known ways to accomplish monitoring/testing of back-up power systems (see supra. Stich et al., Chen, Chen et al., or Collins et al., for example).
However, a battery path test implementation may require that the system being backed-up operate on the back-up battery energy for a period of time to get a realistic full load test. This has been done in the past by disconnecting the main power source and connecting only the back-up battery power, and observing the result. While the load is being supplied with power from the back-up power system, measurements are made to ascertain various parameters, such as the current and voltage capacity of the back-up power system.
However, a problem/limitation of such a procedure, which is particularly relevant where the load is a computer system, for example, is that if the back-up power system does not operate properly during the testing, the computer system (load) might "crash." This is one reason that in the past, a path test was typically done at the beginning of a battery capacity test or when a system is first powered on, i.e., at a predetermined time when precautions can be taken so that the failure of the test has minimal impact on the computer system. In the past, to minimize the risk of a system crash, periodically testing the back-up power system may have required the computer system be taken off-line or shut-down, which is inefficient and uneconomical.
In the past, if, during a path test of a BBU back-up power system, a component in series with the battery failed, the system connected to the back-up power system being tested could crash. (The irony of this should be appreciated, since one purpose of having such a back-up power system in the first place is to avoid a computer system crash in the event of a main power failure.) Computer system crashes are particularly troublesome since there is the risk of loss of valuable data, and because a considerable amount of time is often required to restart a computer system after a crash.
Further, since a fault in the back-up power system may develop at any time, even after the back-up power system passes a start-up test successfully, i.e., before the next test which would detect the fault, and since a main power system failure could occur at any time in the interim, frequent testing, and/or redundant systems, may be required to minimize such a possibility.
As can be readily appreciated, many other programmable (and other) systems besides computer systems could be adversely affected by a failure of a back-up power system during testing. Examples include video cassette recorders (VCR's) and television (TV) sets which may have to be reprogrammed after a loss of power.
The problem is, therefore, that a realistic test requires the load to be operated on the back-up power, but if the back-up power fails during the test, the system being backed-up might be adversely affected, e.g., crash. However, taking a system off-line to test the back-up power system can be costly and inefficient. Also, to minimize the possibility of a failure of the back-up power system in a real emergency, it should be tested as frequently as possible to detect faults in a timely manner.
For the above reasons, it would be desirable to be able to periodically test the back-up power system at any time, even while the backed-up system (computer) is performing critical operations, without risking a crash should the back-up power system fail during the testing.
Therefore, a need exists for a way to test a back-up power system at any time during operation of the equipment being backed-up. This requires eliminating the risk of adversely affecting the equipment being backed-up, such as crashing a computer system, for example, during the testing.
In particular, it is desirable to provide a back-up power system battery path test such that its implementation results in zero risk of affecting a system connected to the back-up power system.